High strength steel for hightemperature service



Patented Aug. 11, 1953 HIGH STRENGTH STEEL FOR HIGH- TEMPERATURE SERVICE Peter Payson, New York, and Joe Gin-Young Chow, Long Island City, N. Y., assignors to Crucible Steel Company of America, New York, N. Y., a corporation of New Jersey No Drawing. Application January 17, 1952, Serial No. 267,003

3 Claims.

This invention relates in general to a forgeable alloy steel for service under high stress at temperatures around 1100 F. for such parts for example, as bolts, studs, flanges and turbine blades in the modern steam power plants and for wheels in gas turbines. It is desirable that such parts have high strength both at room and elevated temperatures.

Although some alloy steels containing appreciable amounts of nickel, cobalt, or columbium have been proposed for such applications, the present scarcity of these metals makes it desirable that a steel be available which has adequate strength and oxidation resistance for service at temperatures up to about 1100 F'., and yet be as free as possible of these strategic elements.

It is the object of this invention to provide a forgeable alloy steel, which is heat treatable to high strength levels; which is adequately resistant to oxidation under long time exposure to temperatures of about 1100 R; which has high strength at these temperatures; and which neither requires nor employs any of the strategically critical elements, nickel, cobalt and columbium.

The steel of the present invention contains as essential alloying additions, chromium, tungsten, molybdenum, vanadium and manganese, with chromium as the major addition, 1. e. about 11 to 14%, together with relatively small amounts of the remaining elements mentioned, as brought out more specifically below.

To provide adequate resistance to oxidation we use a minimum of about 11% chromium inour steel and we may use as much as 14%.

With higher contents of chromium it is diflicult to develop high strength in the steel because of the marked tendency of chromium to inhibit austenite formation, and without such formation, it is not possible to form strength-giving martensite in the steel by heat treatment.

To give the steel strength at elevated temperatures, we use the active carbide forming elements vanadium, tungsten, and molybdenum in combination in relatively small amounts, namely from about 0.20 to .70% vanadium; from about 1.0 to 2.5% tungsten; and from about 1.5 to 4.0% molybdenum. As is well known, these elements in steel cause resistance to softening over the range of temperature from about 900 to 1100 F., and hence provide strength in the steel at these temperatures. The molybdenum content is preferably kept higher than the tungsten content.

Although it has been maintained that tungsten and molybdenum are interchangeable in the ratio of one part tungsten for about 0.6 part molybdenum (see Gill et al., Tool Steels, ASM, 1944, page 510) we have found, as will be shown below, that the combination of both tungsten and molybdenum in our steelimparts higher elevated temperature strength, than is obtainable with just one in suflicient amount to make up for the other. I

Since chromium, vanadium, tungsten, and molybdenum, all are ferrite formers, that is; they tend to inhibit the formation of austenite in the steel, and hence make the steel nonhardenable by ordinary heat treatment, it is necessary to provide in the steel of our invention, compensating austenite formers," viz. carbon and manganese. We find that about 20% carbon along with about 0.50% manganese are sufficient to provide at least austenite in the steel at about 2000 F. when the other elements are present in the amounts mentioned. More than 10% residualferrite is detrimental to the strength of the steel. We find it desirable to limit the carbon to about .40%, and the manganese to about 1.5% on the high side, in order to avoid retention of austenite in the heat treated steel, and to provide adequate ductility in the steel at room temperature after heat treatment.

The silicon in the steel should be sufiicientto assure thorough deoxidation, i. e. on the low side from an effective amount of about 0.1%. And generally-we limit the'silicon to about 0.5% on the high side, since silicon is also an active ferrite former. The broad and preferred ranges of analysis of our steel are as follows:

Analysis, Percent 2.0/2.5. Remainder.

In order to secure the full effect of the alloying elements in the steel for high strength at elevated temperatures, we heat treat with a mini mum austenitizing temperature of 1900 F., and we preferably use 2000" F. The steel is then either oil quenched or air cooled (if the section is relatively small) to room temperature, and then tempered at 1200 to 1300 F., the lower temperature being preferred where the service temperature does. not.. exceedv 1100 F. The mechanical properties. attainable at room. temperature after heat treatment are shown in Table I.

TABLE I Room temperature mechanical propertiesofi steel of this invention.

[Test pieces quenched in oil from ZOQOP-R. and:tempered.;at.l2009;

or 1300 F., ion-Zlionrs] Bar 0 Mn Si CrlV\W Mo Red Area, percent E1. in l.4=in: percent Tensile Strength; p; S. i.

.gTenrperede Rockwell Bar i no TABLE I1? stressmuptwracaluesnat 1100" F.

and 3642, with the same equivalent Mo as Bar 3635 are definitely superior to the latter.

The steel of the invention as ordinarily produced will contain from about .03 to 0.08% nitrogen,,and may advantageously containup to about 0.35% nitrogen, it being understood. that the nitrogen can partially replace the carbon, to the extent that the carbon may be reduced to about 0.1%.

Whatzwe claim. is:

1. A forgeable and heat treatable alloy steel which. as heat treated possesses high strength andgoodductility at room temperature, and high stress-rupture properties at temperatures up to about. 1100 F., said steel having substantially the following composition: 0.2 to 0.4% carbon; 0.5'rto. 115% manganese; 11 to 14% chromium; 0.2"to 0.7% vanadium; 1 to 2.5% tungsten; 1.5 to 4%. molybdenum; from an efiective amount to about 0.5% silicon; remainder iron.

2.v A forgeable and heat. treatable alloy, steel which as heat. treated possesses; high. strength and good ductility at roomtemperature, andhigh stress-rupture properties at temperaturesup. to about 1100 F., said steel having substantially the fol-lowingcomposition: 0.25 to 0.3% carborr; 0.6 to: 0.9% manganese; 0.15 to 014% silicon; 2 to 13% chromium; 0.3 to- 0.51% vanadium; 115 to2% tungsten; 2' to 215% molybdenum; remainder iron.

3. A forgeable and heat treatable alloy'steel which as heat treated possesses" highstrength and good ductility at room temperature, and high stress-rupture properties at temperatures up to about 1100"" K, said, steel. having. substantially the following composition: 0.1 to 0.4% carbon; 0.5 to 1.5% manganese; 11-. to 1.4%. chromium; 0.-2. to 0.7% vanadium; 1 to 2.5% tungsten; L5 to 4-%- molybdenum; from an. eifectivee amount stressed asrindicated] Time to Rube Analysis, Percent Equiva tum, hours Bar 3 1 1 6?,000 56,000 0 Mn Si Cr V W Mo L p; S; L

. 74'- 29 1224.71 38 fl 2230, 23- 6 s .70 27 12.37 37 .75- 1.30 1.8 15 26" 12:32 38 3:29" 353 153 317 73' 27 12'. 63- Y 39 73 1180' 2. 7 128; 334 .75. .25. 12.58- .4]. .75. 2.37 2.8v 138 361 80 29'" 12164 41' 1175" 230 35 3 167' 469 74 26 12. 60 38 l. 74 2; 30; 32' 3- 201 416:

1 Equivalent Mo equals 0.6 X Percent-W plusPercentMm at 1100"'F:; and-the W-bearing steels, Bars 3640 70 2,590,835

to about 0.5% silicon; from an effective amount up to about: 035% nitrogen, the. remainder" iron;

PETER; PAYSON. JOE GIN YoUNG' CHOW.

References Cited in the file ofthis patent UNITED STATES-PATENTS llumber Name Date 2,513,935 Harris July-4,1950 Kirby'et' a1. Apr. 1; 1952 

1. A FORGEABLE AND HEAT TREATABLE ALLOY STEEL WHICH HAS HEAT TREATED POSSESSES HIGH STRENGTH AND GOOD DUCTILITY AT ROOM TEMPERATURE, AND HIGH STRESS-RUPTURE PROPERTIES AT TEMPERATURE UP TO ABOUT 1100* F., SAID STEEL HAVING SUBSTANTIALLY THE FOLLOWING COMPOSITION: 0.2 TO 0.4% CARBON; 0.5 TO 1.5% MANGANESE; 11 TO 14% CHROMIUM; 0.2 TO 0.7% VANADIUM; 1 TO 2.5% TUNGSTEN; 1.5 TO 4% MOLYBDENUM; FROM AN EFFECTIVE AMOUNT TO ABOUT 0.5% SILICON; REMAINDER IRON. 